1. Field of the Invention
This invention in general relates to camera viewfinder systems and in particular, to an improved, periscopic viewfinder system for use with a camera having a zoom lens.
2. Description of the Prior Art
The use of through-the-lens reflex viewfinder arrangements in motion picture, television and other types of cameras is well known. Such arrangements are advantageous because the viewfinder automatically shows the correct field of view irrespective of the focal length of the lens used (especially useful with zoom lenses) and makes visual focusing of the image possible. Commonly, a beamsplitter or small pick-off mirror is positioned in the optical path to direct a percentage of the image forming rays into the viewfinder system which thereafter operates to provide the user with a welldefined, usually rectangular, erect image field more or less equal to that covered by the camera lens.
Generally, such viewfinder arrangements extend along one side of the camera from front to back and, as such, usually comprise a periscopic system which classically consists of an objective, an eyelens or eyepiece and one or more field and erector lenses arranged between the objective and eyelens to extend the length of the system or to erect the image or both.
More recently, perioscopic viewfinder systems have been disclosed which require fewer optical elements than the classical field/relay lens arrangements. The more recent periscopic arrangements eliminate elements by arranging non-spherical elements so that each element performs in part as both objective lens and field lens.
Examples of this more recent type of periscopic viewfinder system are shown, e.g., in U.S. Pat. No. 3,583,785 issued to Jack B. Boardman on June 8, 1971 and entitled "Positive Optical System"; U.S. Pat. No. 3,722,979 issued to Nobunao Mikami on Mar. 27, 1973 and entitled "Optical System of the Real Image Type for Finders Having Aspheric Surfaces"; U.S. Pat. No. 3,792,650 issued to Arthur C. Mueller et al. on Feb. 19, 1974 and entitled "Camera Focus Indicia System"; and U.S. Pat. No. 4,270,837 issued to James G. Baker on June 2, 1981 and entitled "Periscopic Viewfinder System For Use In Photographic Cameras."
It is a primary object of the present invention to provide an improved periscopic viewfinder for use in an electronic imaging camera having a zoom lens.
Other objects of the invention in part will be obvious and in part will appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts which are exemplified in the following detailed disclosure.
SUMMARY OF THE INVENTION
The invention is a viewfinder for use in an electronic still camera that has a preceding afocal or nearly afocal zoom lens. The viewfinder itself is an elongated, periscopic type that is folded and comprises a pickoff mirror located near the camera system stop, a two-element objective, a dichroic beamsplitter which reflects visible and transmits infrared, a field lens, a relay, and an eyepiece. Behind the beamsplitter is an infrared photodetector.
The pickoff mirror is in a nearly collimated ray bundle emerging from the zoom and operates to divert part of the light for travel along a folded path to the objective. The objective forms a real image of the scene in a plane in or nearly in the field lens. The real image is relayed by a symmetrical pair of new achromats to an intermediate image location where it is viewable by the eyepiece. The stop of the relay system is imaged onto the pickoff mirror. The new achromats of the relay do not appreciably add to the field curvature introduced by the positive elements leading it, and the entire system includes glass and plastic elements and aspheric surfaces for aberration control.